The present invention relates to a magnetic recording medium and a process for producing the same, and more particularly, to a magnetic recording medium composed of a perpendicular magnetic film which has an excellent oxidation resistance, an excellent corrosion resistance, an appropriate coercive force for preventing a magnetic saturation of a magnetic head which is widely used at present, particularly, a coercive force of less than 3000 Oe and a large squareness (value compensated by a demagnetization coefficient) and hence, which is suitable as a high-density magnetic recording medium, and a process for producing the magnetic recording medium.
With recent remarkable tendency for downsizing and higher reliability of information processing apparatuses and systems, magnetic recording media have increasingly been required to have a higher recording density. Perpendicular magnetic films as magnetic recording media which respond to such demand have been rapidly developed and put to practical use. Perpendicular magnetic films which are magnetized perpendicularly to the film plane are not only free from demagnetization but also capable of high-density recording.
An alloy film such as a CoCr alloy film has conventionally been proposed as a perpendicular magnetic film. However, it Is necessary to coat the surface of a CoCr alloy film with a carbon film having a thickness of about 100 to 200 .ANG. in order to prevent the deterioration of the magnetization characteristics. As a result, the spacing loss caused by the distance between a magnetic head and the recording layer is increased by the distance corresponding to the thickness of the carbon film, which is unsuitable for high-density recording. For this reason, a material of a perpendicular magnetic film is strongly required to be an oxide which is stable against oxidation.
As to the coercive force of a perpendicular magnetic film, an appropriate coercive force for preventing a magnetic saturation of a magnetic head which is widely used at present, particularly, a coercive force of less than 3000 Oe is required.
The coercive force Hc of a magnetic recording medium has a close relationship with the performance of a magnetic head, as is well known.. When the coercive force Hc of the magnetic recording medium is as high as more than 3000 Oe, the current at which a writing operation is performed becomes so high that the head core of a magnetic head which is widely used at present, is magnetically saturated due to an insufficient saturation flux density Bm. As a result, it is impossible to sufficiently magnetize the magnetic recording medium.
A ferrite head is widely used in a magnetic recording and reproducing system which corresponds to a magnetic recording medium having a coercive force of not more than 1000 Oe, while a head such as a Sendust head, an amorphous head and a thin-film head, whose head core is made of a material having a high saturation flux density, is used in a magnetic recording and reproducing system which corresponds to a magnetic recording medium having a coercive force of more than 1000 Oe.
In addition, a magnetic recording medium is required to have as large a reproduction output as possible. For this reason, a perpendicular magnetic film is required to have as large a squareness as possible.
As a perpendicular magnetic film for magnetic recording, an alloy film such as a CoCr alloy film and a CoPt alloy film, a spinel oxide thin film such as a cobalt ferrite film (Japanese Patent Application Laid-Open (KOKAI) Nos. 51-119999 (1976), 63-47359 (1988), 3-17813 (1991), 3-188604 (1991), 4-10509 (1992), and 5-12765 (1993)), a magneto-plumbite oxide thin film such as a barium ferrite film (Japanese Patent Application Laid-Open (KOKAI) No. 62-267949 (1987)) and the like have conventionally been proposed.
Among the above-described perpendicular magnetic films, the cobalt ferrite (Co.sub.x Fe.sub.3-x O.sub.4) films which are typical of spinel oxides are stable against oxidation, because the cobalt ferrite (Co.sub.x Fe.sub.3-x O.sub.4) films are oxides, and have a large crystalline magnetic anisotropy. Owing to these magnetic characteristics the cobalt ferrite films are considered to be promising as a perpendicular magnetic recording medium.
As the process for producing-a cobalt ferrite (Co.sub.x Fe.sub.3-x O.sub.4) film, various methods such as sputtering method, vacuum evaporation method and MO-CVD method are known.
In order to improve the orientation of a perpendicular magnetic film, various attempts have recently been made. For example, a single crystal material is used as a substrate, and various types of primary-layers are formed between a perpendicular magnetic film and a substrate. There are known a perpendicular magnetic film using a MgO single crystal as a substrate (IEEE Trans. Mag. MAG-12, No. 6,733 (1976), IEEE Trans. Mag. MAG-14, No. 5,906 (1978) and Czehch. J. Phys. B21, 563 (1971)), a perpendicular magnetic film using NaCl layer as a substrate (J. Cry. Growth. 50, 801 (1980)), a perpendicular magnetic film using NiO layer as an primary film (Japanese Patent Application Laid-Open (KOKAI) No. 5-166167 (1993)), etc.
Although a perpendicular magnetic film which has an excellent oxidation resistance, an excellent corrosion resistance, and an appropriate coercive force for preventing a magnetic saturation of a head which is widely used at present, is now in the strongest demand, none of the conventional magnetic thin films sufficiently meet these requirements.
For example, a cobalt ferrite (Co.sub.x Fe.sub.3-x O.sub.4) film produced by a sputtering method is disadvantageous in that although the easy magnetization axis of a cobalt ferrite (Co.sub.x Fe.sub.3-x O.sub.4) film is an axis (100), the axis (100) thereof is likely to orient at random or the plane (111) is likely to orient in parallel with the surface of the substrate, so that it is difficult to produce a perpendicular magnetic film. (1). The method described in Proceedings of the 9-th Meeting of Magnetic Society of Japan 29PB-10, (2) the method described in Proceedings of the 13-th Meeting Magnetic Society of Japan, p 246, and (3) the method described in Japanese Patent Application Laid-Open (KOKAI) No. 4-10509 (1992) are known as examples of a method for obtaining a cobalt ferrite (Co.sub.x Fe.sub.3-x O.sub.4) film in which the plane (400) is predominantly oriented in parallel with the surface of the substrate.
The method (1) is a method of depositing Fe and Co ionized in an oxygen plasma on an MgAl.sub.2 O.sub.4 substrate or a silica glass substrate heated to 500.degree. C. Since it is necessary to maintain the substrate temperature at a high temperature such as not lower than 500.degree. C. during film formation, the productivity is poor. In addition, in order to raise the substrate temperature to not Lower than 500.degree. C., the substrate itself is required to have a high heat resistance. However, the heat resistance of glass or the like, which is widely used as a material of the substrate for a perpendicular magnetic recording medium, is insufficient. In this way, since the material of the substrate is limited, it is not advantageous either industrially or economically.
The method (2) is a plasma-exciting MO-CVD method. Since it is necessary to maintain the substrate temperature at 300.degree. to 400.degree. C. in a vacuum during film formation, the productivity is poor, which is industrially and economically disadvantageous.
The method (3) is a method of annealing a multilayered metal film produced by laminating at least two layers of Co and Fe at a temperature of not lower than 500.degree. C. in an atmosphere containing oxygen. Since a high temperature is necessary, the material of the substrate is limited, as described above, which is disadvantageous industrially and economically.
If an MgO single crystal layer or NaCl layer is used as a primary film as in the known technique, it is easy to form a ferrite film as a perpendicular film in which the plane (400) is substantially oriented in parallel with the surface of the substrate, but since the MgO single crystal layer is expensive, the MgO single crystal layer and NaCl layer are easily broken and it is difficult to obtain the MgO single crystal layer or NaCl layer having a large area, use of such the MgO single crystal layer or NaCl layer as a substrate is not practical.
Since an NiO film as a primary film in which the plane (200) is substantially oriented in parallel with the surface of the substrate can be easily produced on a glass substrate by sputtering at room temperature, it is practical to use the film as a primary film. However, the perpendicular magnetic film described in Japanese Patent Application Laid-Open (KOKAI) No. 5-166167 (1993) arises the problem that although the crystal orientation of the plane (400) is accelerated by forming Co ferrite on the NiO primary film. As seen from Japanese Patent Application Laid-Open (KOKAI) No. 3-17813 (1991), when the lattice constant of a primary film is larger than that of a ferrite film, the perpendicular anisotropy of the ferrite film becomes larger. But when a Co-ferrite film is disposed on an NiO primary film, the lattice constant of the primary film is smaller than that of the Co-ferrite film. Namely, since the spacing (2.09 .ANG.) of the plane (200) of the NiO primary film is smaller than the spacing. (2.10 .ANG.) of the plane (400) of Co.sub.x Fe.sub.3-x O.sub.4, the perpendicular anisotropy is reduced due to the compressive stress in the plane direction of the Co-ferrite film.
European Patent No. 0 586 142 Al discloses a perpendicular magnetic film comprising: a primary layer of an NiO film formed on a substrate, in which the plane (100) is predominantly oriented in parallel with the substrate; and a Co-containing .gamma.-Fe.sub.2 O.sub.3 film formed on the primary layer, in which the plane (400) is predominantly oriented in parallel with the substrate, the molar ratio of Co to Fe is from 0.10:1 to 0.32:1, the spacing of the plane (400) is not more than 2.084 .ANG., and the optical absorption coefficient at 700 nm is not more than 2.5 .mu.m.sup.-1.
The perpendicular magnetic film disclosed in European Patent No. 0 586 142 Al has a coercive force of not less than 4000 Oe, and the technical problem thereof is to provide a perpendicular magnetic film which has an excellent oxidation resistance, an excellent corrosion resistance, a large coercive force, e.g., a coercive force of not less than 4000 Oe, a large squareness, a large Faraday rotation angle in a short-wavelength region, and a small optical absorption coefficient.
The technical problem to be solved by the present invention is to produce a perpendicular magnetic film which has an excellent oxidation resistance, an excellent corrosion resistance, an appropriate coercive force, for example, a coercive force of less than 3000 Oe (in which the technical developing direction is just in the opposite direction to that of European Patent No. 0 586 142 Al), at a temperature which is lower than 500.degree. C. and as low as possible, under the industrially and economically favorable conditions.
As a result of studies undertaken by the present inventor so as to solve the technical problem, it has been found that by forming, on the surface of a substrate, an NiO primary layer in which the plane (200) is predominantly (substantially) oriented in parallel with the surface of the substrate; forming a monolayered Co-containing magnetite film or a multilayered film composed of at least one unit on the NiO primary layer, one unit being a laminate of a magnetite layer and a CoO layer, in which the plane (400) is predominantly (substantially) oriented in parallel with the surface of the substrate and the molar ratio of Co to Fe is not less than 0.01 and less than 0.10; and annealing the monolayered film or multilayered film at 240.degree. to 450.degree. C., the thus obtained perpendicular magnetic film has a coercive force of less than 3000 Oe, a large squareness (value compensated by a demagnetization coefficient) such as not less than 0.88, an excellent oxidation resistance and an excellent corrosion resistance. On the basis of this finding, the present invention has been achieved.